This application is based on, and claims priority of, Canadian Patent Application No. 2,326,362, filed Nov. 20, 2000, Canadian Patent Application No. 2,327,862, filed Dec. 6, 2000, and Canadian Patent Application No. 2,328,756, filed Dec. 19, 2000.
Not Applicable.
The present invention relates to optical cross-connect switches, and in particular to a control system for an optical cross connect capable of detecting and correcting mirror positioning errors within the optical cross-connect.
Optical matrix cross-connects (or switches) are commonly used in communications systems for transmitting voice, video and data signals. Generally, optical matrix cross-connects include multiple input and/or output ports and have the ability to connect, for purposes of signal transfer, any input port/output port combination, and preferably, for Nxc3x97M switching applications, allow for multiple connections at one time. At each port, optical signals are transmitted and/or received via an end of an optical waveguide. The waveguide ends of the input and output ports are optically connected across a switch core. In this regard, for example, the input and output waveguide ends can be physically located on opposite sides of a switch core for direct or folded optical path communication therebetween, in side-by-side matrices on the same physical side of a switch core facing a mirror, or they may be interspersed in a single matrix arrangement facing a mirror.
Establishing a connection between an input port and a selected output port involves configuring an optical path across the switch core. One known way to configure the optical path involves the use of one or more movable mirrors interposed between the input and output ports. In this case, the waveguide ends remain stationary and the mirrors are used to deflect a light beam propagating through the switch core from the input port to effect the desired switching. Micro-electro-mechanical mirrors known in the art can allow for one- or two-dimensional targeting to optically connect any input port to any output port. For example, U.S. Pat. No. 5,914,801, entitled MICROELECTROMECHANICAL DEVICES INCLUDING ROTATING PLATES AND RELATED METHODS, which issued to Dhuler et al on Jun. 22, 1999; U.S. Pat. No. 6,087,747, entitled MICROELECTROMECHANICAL BEAM FOR ALLOWING A PLATE TO ROTATE IN RELATION TO A FRAME IN A MICROELECTROMECHANICAL DEVICE, which issued to Dhuler et al on Jul. 11, 2000; and U.S. Pat. No. 6,134,042, entitled REFLECTIVE MEMS ACTUATOR WITH A LASER, which issued to Dhuler et al on Oct. 17, 2000, disclose micro-electro-mechanical mirrors that can be controllably moved in two dimensions to effect optical switching.
One of the major challenges of designing an optical cross-connect (OXC) switch using tiltable Micro-Electro-Mechanical Switch (MEMS) mirrors is the need to accurately control each of the mirrors so that low fiber-to-fiber losses can be maintained over the operation lifetime of the switch. The major obstacle to creating an optical switch is the necessary control for precisely addressing each of the mirrors to achieve accurate switching with low loss. Small errors in angle over the optical path length of the switch can easily result in large coupling errors.
U.S. Pat. No. 6,097,858, entitled SENSING CONFIGURATION FOR FIBER OPTIC SWITCH CONTROL SYSTEM, and U.S. Pat. No. 6,097,860, entitled COMPACT OPTICAL MATRIX SWITCH WITH FIXED LOCATION FIBERS, both of which issued to Laor on Aug. 1, 2000, disclose switch control systems for controlling the position of two-dimensionally movable mirrors in an optical switch. Laor discloses a complex control system for detecting angle deviation. Because the optical path includes first and second reflections (in a Z pattern) between launching a focused beam and coupling a switched beam to a selected output port, a cumulative error will be detected at the output. That is, the coupling error of the switched beam into the output port will be the aggregate of the angular positioning errors of both of the involved mirrors. Determination of the angle error of each mirror is complex and difficult.
Accordingly, a control system for an optical cross connect, in which angle position errors of each involved mirror is unambiguously detected and controlled, remains highly desirable.
Accordingly, an object of the present invention is to provide a control system for an optical cross connect, in which angle position errors of each involved mirror is unambiguously detected and controlled.
Thus an aspect of the present invention provides a control system for an optical cross-connect having a switch core defined by a pair of opposed MEMS mirror arrays designed to selectively define an optical path between a pair of waveguides of the optical cross-connect. The control mechanism includes an optical element having optical power disposed in the optical path between the MEMS arrays; a respective optical sensor associated with each MEMS mirror; and a feedback control between the optical sensor and its associated MEMS mirror.
Due to the location of the optical element having optical power, a light beam switched through the cross-connect encounters the optical element having optical power three times: a first encounter between the input waveguide and a first MEMS mirror; a second encounter between the first MEMS mirror and a second MEMS mirror in the opposite MEMS array; and a third encounter between the second MEMS mirror and the output waveguide. As a result, positioning errors of each involved mirror cause characteristic perturbations in geometric properties of the light beam arriving at the output waveguide, and these perturbations can be unambiguously related to the specific mirror in question. For example, a positioning error of the first mirror causes a lateral offset of the propagation path of the light beam arriving at the output waveguide, while a positioning error of the second mirror causes an angular offset of the propagation path of the light beam arriving at the output waveguide. It is therefore possible to unambiguously relate geometric properties (angle or lateral position) of the path of light beams arriving at the output waveguide to a specific mirror.
Thus each optical sensor is designed to detect a predetermined geometric property (i.e., either lateral or angular position) of a respective light beam arriving at an associated waveguide from a respective MEMs mirror. The feedback control can then actively control the respective mirror, based on the detected geometric property, to optimize coupling of the light beam into the waveguide.
Advantageously, one wavefront sensor and feedback control is provided for each mirror. Each mirror of each array can therefore be checked and corrected, simultaneously, in real time.